1. Field of the Invention
The present invention relates to a process for the production of Grignard compounds, according to which magnesium is reacted in a suitable liquid reaction medium under a protective gas atmosphere with halosubstituted organic compounds with microwave irradiation.
Over the past few decades, organometallic compounds have proven to be an indispensable reaction component in the synthesis of organic compounds both in the laboratory and in large industrial scale production despite their sometimes high sensitivity to atmospheric oxygen and moisture.
2. Brief Description of Related Developments
Organomagnesium compounds, which also include so-called Grignard compounds, have gained particular significance. These Grignard compounds may be obtained by reacting magnesium with halosubstituted organic compounds under a protective gas atmosphere in a suitable reaction medium and comprise a strongly polarised magnesium-carbon bond, in which the carbon atom bears the negative charge density, see for example, “Grignard Reagents: New Developments”, Herman G. Richey Jr., John Wiley and Sons Ltd, 2000, pages 185 to 275, “Organomagnesium Methods in Organic Synthesis”, Basil J. Wakefield, Academic Press, London, 1995, pages 21 to 71 and “Handbook of Grignard Reagents”, Gary S. Silverman, Philip E. Rakita, Marcel Dekker, Inc., New York, 1996, pages 53 to 77.
The Grignard compounds therefore readily react as nucleophilic reagents with electrophilic compounds with formation of a new carbon-carbon-bond and are thus suitable for synthesis of larger organic compounds from appropriate fragments.
In practice, the production of Grignard compounds frequently proves difficult, as the surface of the magnesium is conventionally passivated by a magnesium oxide layer, which markedly impairs or even prevents reaction of the magnesium with the halosubstituted organic compound.
Processes have therefore been developed to activate the surface of the magnesium. Examples are chemical activation by mineral acids, elemental iodine or and by lithium salts, together with physical activation by heating or by ultrasound treatment.
However, a disadvantage of these processes for activating the magnesium is that, due to the long reaction times and optionally the elevated reaction temperatures, undesired secondary products are often formed in addition to the desired Grignard compound, such as for example hydrocarbon compounds by the reaction of two halosubstituted organic compounds through Wurtz coupling.
These secondary reactions lead on the one hand to a reduction in the yield of the desired Grignard compound and thus also to a reduction in the yield of the derived product produced therefrom. On the other hand, these secondary reactions frequently also impair or prevent the use of Grignard compounds in a synthesis, since the subsequently necessary purification steps make the process uneconomic or cannot be performed at all due to small substance quantities, such as for example in the field of combinatorial chemistry.